Exhaust gas system for an internal combustion engine

ABSTRACT

An exhaust gas system for an internal combustion engine has a thermoelectric power generator with a hot side and a cold side. The hot side is arranged on the exhaust gas system and is heatable by exhaust gas of the internal combustion engine. A first heat exchanger having a first thermal conductivity is arranged between the hot side and the exhaust gas flow. At least a second thermoelectric power generator is arranged on the exhaust gas system and has a second heat exchanger with a thermal conductivity that is different from the first thermal conductivity. The arrangement of thermoelectric power generators can generate electric power over a significantly wider operating point range of the internal combustion engine.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT International Application No.PCT/EP2009/002885, filed Apr. 21, 2009, which claims priority under 35U.S.C. §119 from German Patent Application No. DE 10 2008 023 831.7,filed May 15, 2008, the entire disclosures of which are herein expresslyincorporated by reference.

This application contains subject matter related to U.S. applicationSer. No. 12/944,832, entitled “Cooling System for a Thermoelectric PowerGenerator (TEG)” filed on even date herewith.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to an exhaust gas system for an internalcombustion engine having a thermoelectric power generator with a hotside and a cold side, the hot side being arranged on the exhaust gassystem and being heatable by the exhaust gas of the internal combustionengine. A first heat exchanger having a first thermal conductivity isarranged between the hot side and the exhaust gas flow.

The relevant prior art includes, for example, published European patentapplication EP 1 475 532 A2, which discloses an internal combustionengine having an intake system and an exhaust gas system. The exhaustgas system and the intake system are connected together in agas-conveying manner by way of an exhaust gas recirculation device. Theexhaust gas recirculation device has a thermoelectric power generator,in order to recover electric power from the thermal energy of theexhaust gas. In order to guarantee an optimal supply of heat to thethermoelectric power generator by means of the exhaust gas recirculationline, valves are arranged in the exhaust gas recirculation line upstreamand downstream of the thermoelectric power generator. These valves areactuated by a control unit as a function of the operating point. Inorder to obtain an optimal temperature differential between the hot sideand the cold side of the thermoelectric power generator, the cold sideof the thermoelectric power generator is connected to a coolant circuit.

Furthermore, it is known from published Japanese patent application JP7012009 A2, on which this invention is based, to arrange thethermoelectric power generators on the exhaust gas system of an internalcombustion engine. In this case, the exhaust gas line is divided into aplurality of parallel branches that convey the exhaust gas. Each ofthese branches has two thermoelectric power generators, as a result ofwhich an optimal electric power generation under minimum exhaust gasback pressure conditions can be achieved. Furthermore, this branchedexhaust gas system makes it possible to significantly reduce the amountof design space.

The drawback with the prior art arrangement of thermoelectric powergenerators is the relatively narrow speed or, more particularly, theload range of the internal combustion engine, in which thethermoelectric power generators optimally function, because theefficiency is very highly dependent on the temperature of the exhaustgas. Moreover, there is the risk of overheating if the internalcombustion engine is operated in a high speed (load range) for aprolonged period of time.

An object of the present invention is to avoid the above-describeddrawbacks and to improve the efficiency of the electric power generationwith thermoelectric power generators.

This and other objects are achieved with an exhaust gas system that isintended for an internal combustion engine and that includes athermoelectric power generator with a hot side and a cold side, the hotside being arranged on the exhaust gas system and being heatable by theexhaust gas of the internal combustion engine. A first heat exchangerhaving a first thermal conductivity is arranged between the hot side andthe exhaust gas flow. At least a second thermoelectric power generatoris arranged on the exhaust gas system and includes a second heatexchanger having a thermal conductivity that is different from the firstthermal conductivity.

In the simplest embodiment, the thermoelectric power generator havingthe heat exchanger with the lower thermal conductivity is arranged inclose proximity to the internal combustion engine, and the secondthermoelectric power generator having a heat exchanger with the higherthermal conductivity is arranged away from the internal combustionengine. This measure prevents the first thermoelectric power generatorfrom being destroyed by overheating with an exhaust gas that is too hot.At the same time, the second thermoelectric power generator, includingthe heat exchanger that has the higher thermal conductivity and isoperatively connected to the colder exhaust gas that has already beencooled, exhibits a higher efficiency at lower temperatures owing to thestrategy according to the invention.

Preferably, in one embodiment, the exhaust gas system is characterizedin that the exhaust gas system has at least a first and a second bypass,which are arranged one after the other in the direction of flow of anexhaust gas, the first thermoelectric power generator being arranged inthe first bypass in the direction of flow of the exhaust gas, and thesecond thermoelectric power generator being arranged in the secondbypass, and wherein a closure element for the exhaust gas is provided inat least one bypass.

Preferably, in the exhaust gas system, the first thermoelectric powergenerator has the heat exchanger with the lower thermal conductivity,and the second thermoelectric power generator has the heat exchangerwith the higher thermal conductivity. This makes it possible to achievea very good efficiency in the course of generating electric power.

In a second especially preferred design variant, the first and thesecond thermoelectric power generators are arranged in parallel to eachother in the bypass, and wherein each thermoelectric power generator isheatable by the exhaust gas, at least one closure element for theexhaust gas being assigned to at least one thermoelectric powergenerator.

In another embodiment that makes it possible for the second especiallypreferred design variant to achieve a very good efficiency in the courseof generating electric power, the exhaust gas of the internal combustionengine exhibits a colder or hotter temperature that is a function of anoperating point. At least one closure element can be switched in such away that hotter exhaust gas can be conveyed to the thermoelectric powergenerator having the heat exchanger with the lower thermal conductivity.

The use of a control unit is especially preferred. If the control unitis a control unit of the internal combustion engine, then this controlunit also allows the characteristic parameters to be controlled in anadvantageous manner.

In yet another embodiment that achieves an optimal temperature gradientbetween the hot side and the cold side of the thermoelectric powergenerator, the cold side of the thermoelectric power generator isintegrated into a coolant circuit.

In an embodiment that achieves a very fast temperature rise of thethermoelectric power generators, the exhaust gas system has anadditional closure element, wherein the additional closure element canbe controlled in such a manner that all of the exhaust gas flows throughthe bypass.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of one ormore preferred embodiments when considered in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a first inventive embodiment for anexhaust gas system for an internal combustion engine; and

FIG. 2 is a schematic diagram of a second inventive embodiment for anexhaust gas system for an internal combustion engine.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following, the same reference numerals apply to the samecomponents in both figures.

The term thermoelectric power generator (TEG) is defined hereinafter asa semiconductor component, which generates electric power at atemperature differential between a hot side and a cold side. Sincethermoelectric power generators are already sufficiently well-known fromthe prior art, they are not explained in detail in the presentapplication. Moreover, the term thermoelectric power generator may bedefined as a single semiconductor component or as an interconnection ofseveral individual semiconductor components so as to form a packet.

FIG. 1 is a schematic diagram of a first embodiment of an inventiveexhaust gas system 1 for an internal combustion engine 2. An initiallydual pipe exhaust gas system 1 is arranged on the internal combustionengine 2, each pipe being assigned an exhaust gas purification system10. In the direction of flow of an exhaust gas of the internalcombustion engine 2, downstream of the exhaust gas purification systems10, the exhaust gas system 1 is designed as a single pipe. Then, theexhaust gas system has a first bypass 1′, in which a firstthermoelectric power generator 3 is arranged. Downstream of the firstthermoelectric power generator 3, a closure element 5 is arranged in thebypass 1′. Downstream of the closure element 5, the first bypass 1′empties again in a gas-conveying manner into the exhaust gas system 1.Then, the exhaust gas system 1 has a second bypass 1″, in which a secondthermoelectric power generator 4 is arranged. Downstream of the secondthermoelectric power generator 4, a closure element 5 is also arrangedin the second bypass, before the second bypass 1″ empties again in agas-conveying manner into the exhaust gas system 1. Then in thedirection of flow of the exhaust gas, first a central muffler (MSD) andthen a rear muffler (NSD) are arranged in the exhaust gas system 1.Furthermore, a closure element 5′ for the exhaust gas system 1 isarranged in parallel to the second bypass 1″ in the exhaust gas system1.

The closure elements 5, 5′ can be controlled by way of a control unit 6,for example, a control unit of an internal combustion engine, preferablyas a function of the operating point of the internal combustion engine,by use of a characteristic map. Whereas the hot sides of thethermoelectric power generators 3, 4 are arranged on the exhaust gassystem 1 and can be heated by the exhaust gas of the internal combustionengine 2, the cold sides of the thermoelectric power generators 3, 4 inthe present embodiment are operatively connected to a coolant circuit 7.The coolant circuit 7 has a coolant pump 8 for conveying coolant and twoheat exchangers 9 for cooling the coolant.

Each thermoelectric power generator 3, 4 has between its hot side andits cold side a heat exchanger having a defined thermal conductivity.The thermal conductivity of the thermoelectric power generator 3 that islocated in close proximity to the internal combustion engine 2 is lessthan that of the heat exchanger of the second thermoelectric powergenerator 4. Owing to the inventive configuration of the exhaust gassystem 1, the first thermoelectric power generator 3 is driven with anexhaust gas that is hotter than that of the second thermoelectric powergenerator 4. Due to the varying thermal conductivities of the heatexchangers, the effect, that is, the cooling of the exhaust gas over thelength of the exhaust gas system 1, is compensated, thus significantlyincreasing the efficiency for the recovery of electric power of thewhole arrangement.

If the exhaust gas becomes too hot at certain operating points of theinternal combustion engine 2, then the closure element 5 is closeddownstream of the first thermoelectric power generator 3, so that anoverheating is absolutely ruled out. At this point, the hot exhaust gasis conveyed only through the second thermoelectric power generator 4,which can still be used to its full capacity since the exhaust gas hascooled down in the interim. If the speed and/or the load of the internalcombustion engine is (are) raised even more, then the exhaust gas massflow through the second thermoelectric power generator 4 can also beswitched off with the closure element 5 downstream of the secondthermoelectric power generator 4. Consequently, both thermoelectricpower generators 3, 4 are reliably protected from overheating. With theclosure element 5′ in the exhaust gas system 1, the entire exhaust gasmass flow can be passed in an advantageous way through thethermoelectric power generators 3, 4 for the fastest possibletemperature rise in order to generate electric power.

Additional embodiments can provide even more thermoelectric powergenerators in additional bypasses, which in turn have heat exchangersexhibiting different thermal conductivities. It is also possible undercertain conditions to dispense with the coolant circuit 7, if, forexample, an air cooling system is provided. It is also possible toreduce the two heat exchangers 9 to a single large heat exchanger orseveral smaller heat exchangers. Moreover, the exhaust gas purificationsystems 10 in other embodiments can be integrated at other locations inthe exhaust gas system 1.

In additional embodiments, the arrangement of the closure elements 5, 5′can also be configured in such a manner that they are arranged, forexample, upstream of the thermoelectric power generators 3, 4 in thedirection of flow of the exhaust gas. Some examples of closure elements5, 5′ that may be used are exhaust gas flaps or rotary slide valves.Preferably the exhaust gas system 1 and the bypasses 1′, 1″ exhibit thesame exhaust gas flow cross section in order to minimize the flowlosses.

FIG. 2 is a schematic diagram of a second especially preferredembodiment of an inventive exhaust gas system 1 for the internalcombustion engine 2. The second especially preferred embodiment differsfrom the first especially preferred embodiment in FIG. 1 in that thethermoelectric power generators 3, 4, 11 (three thermoelectric powergenerators are used in this embodiment) are arranged in parallel to eachother in a single, bifurcated bypass 1′″. As in the case of the firstespecially preferred embodiment, in the second especially preferredembodiment, the thermoelectric power generators 3, 4, 11 also have heatexchangers exhibiting different thermal conductivities. Consequently,the exhaust gas can be conveyed to one or also two or, if the closureelement 5′ is closed in the exhaust gas system 1, also through all threethermoelectric power generators 3, 4, 11, by use of the closure elements5, which are arranged downstream of the thermoelectric power generators3, 4, 11 in the direction of flow of the exhaust gas, according to theoperating state of the internal combustion engine, that is, as afunction of the speed and/or of the demanded torque, thus according tothe exhaust gas temperature. Should the exhaust gas become too hot dueto a high load or speed owing to the operating state of the internalcombustion engine, so that at least one thermoelectric power generator3, 4, 11 could be destroyed by the heat, the closure elements 5 can becompletely closed. Then the hot exhaust gas flows only through theexhaust gas line 1, as described in conjunction with the firstembodiment. Consequently, the thermal destruction of the thermoelectricpower generators 3, 4, 11 is absolutely ruled out. Furthermore, all ofthe aforesaid with respect to the first especially preferred embodimentalso applies to the second especially preferred embodiment.

A practical arrangement of the second especially preferred embodimentcould be as follows, for example:

First TEG 3: Very aggressive hot gas heat exchanger (for example, highrib density), maximum power output of the thermoelectric power generator3 in a range between 30 and 70 km/h of a vehicle equipped with theexhaust gas system 1.

Second TEG 4: Power output of the hot gas heat exchanger is reducedcompared to stage 1 (for example, reduced rib density, rib type),maximum power output of the thermoelectric power generator in a rangebetween 70 and 130 km/h of the vehicle equipped with the exhaust gassystem 1.

Third TEG 11: Very weak hot gas heat exchanger, maximum power output ofthe thermoelectric power generator in a range between 130 and 180 km/hof the vehicle equipped with the exhaust gas system 1.

On the whole, the inventive designs of the exhaust gas system 1 offerthe following advantages:

(a) Spectrum of use that is significantly wider than that of a singlestage TEG.

(b) Owing to an optimized low temperature TEG (heat exchanger having avery good thermal conductivity) also use in city traffic and/or at lowload phases (speed range between approximately 30 and approximately 70km/h).

(c) Owing to an optimized high temperature TEG (heat exchanger having avery low thermal conductivity) thermal recuperation even at relativelyhigh speeds possible, no overload of the thermoelectric material, noback pressure problem due to a weak hot gas heat exchanger (speed rangebetween approximately 130 and approximately 180 km/h).

(d) Optimization of the thermoelectric power generator in mid speedrange (speed range between approximately 70 and approximately 130 km/h)with the goal of achieving an optimal total utility in the speed windowthat is driven the most.

TABLE OF REFERENCE NUMERALS

1. exhaust gas system

1′ first bypass

1″ second bypass

1′″ third bypass

2. internal combustion engine

3. first TEG

4. second TEG

5. closure element

5′ closure element exhaust gas system

6. control unit

7. coolant circuit

8. coolant pump

9 heat exchanger

10. exhaust gas purification system

11. third TEG

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. An exhaust gas system for an internal combustionengine, comprising: a first thermoelectric power generator having afirst heat exchanger with a first thermal conductivity, the firstthermoelectric power generator having a hot side and a cold side withthe hot side being arrangeable on the exhaust gas system so as to beheatable by exhaust gas of the internal combustion engine, wherein thefirst heat exchanger is arranged between the hot side and the exhaustgas flow; a least a second thermoelectric power generator having asecond heat exchanger with a second thermal conductivity different fromthe first thermal conductivity of the first heat exchanger, the secondthermoelectric power generator being arrangeable on the exhaust gassystem; and a first bypass and a second bypass, the first and secondbypasses being arranged one after the other in an exhaust gas flowdirection; at least one closure element for the exhaust gas arranged inat least one of the first bypass and the second bypass; and acontroller, wherein the first thermoelectric power generator is arrangedin the first bypass in the exhaust gas flow direction, and the secondthermoelectric power generator is arranged in the second bypass, and thecontroller is configured to operate the closure element based on anindication of an energy content of the exhaust gas to optimizethermoelectric power generation while preventing overheating of thefirst and second thermoelectric power generators.
 2. The exhaust gassystem according to claim 1, wherein the first heat exchanger of thefirst thermoelectric power generator has a lower thermal conductivitythan the second heat exchanger of the second thermoelectric powergenerator.
 3. The exhaust gas system according to claim 1, whereinexhaust gas of the internal combustion engine exhibits a colder orhotter temperature as a function of an operating point; and wherein theclosure element is switchable such that hotter exhaust gas is conveyedto the one of the first and second thermoelectric power generatorshaving the respective first and second heat exchanger with a lowerthermal conductivity than the other of said first and second heatexchangers.
 4. The exhaust gas system according to claim 3, furthercomprising a control unit operatively configured to drive the closureelement.
 5. The exhaust gas system according to claim 1, furthercomprising a coolant circuit in which the cold side of the first andsecond thermoelectric power generators are integrated.
 6. The exhaustgas system according to claim 2 further comprising a coolant circuit inwhich the cold side of the first and second thermoelectric powergenerators are integrated.
 7. The exhaust gas system according to claim3, further comprising a coolant circuit in which the cold side of thefirst and second thermoelectric power generators are integrated.
 8. Theexhaust gas system according to claim 2, further comprising anadditional closure element, the additional closure element beingcontrollable such that all exhaust gas flows through the first andsecond bypasses.
 9. The exhaust gas system according to claim 3, furthercomprising an additional closure element, the additional closure elementbeing controllable such that all exhaust gas flows through the first andsecond bypasses.